The present invention relates to the acquisition and conversion of optical signals to electrical signals and more particularly to a method of compensating for relaxation transients in an optical receiver using an optical modulator automatic masking system.
Optical time domain reflectometers, OTDR, are used for testing optical transmission lines, such as fiber optic cables and the like, for attenuation, discontinuities, and faults, which affect the quality of optical signals transmitted through the fiber. In testing an optical fiber with an OTDR, the fiber under test is connected to a front panel connector and optical pulses from a laser are launched into the fiber. During the interval between the pulses, backscattered light from the fiber produced by the Rayleigh effect and light produced by discrete reflection sites are directed to a photosensitive detector, such as an avalanche photodiode or the like. The detector converts the backscattered light signal into an electrical signal, which is amplified, sampled and displayed on an output device. Discontinuities associated with cable splices and the like may produce return reflections that are orders of magnitude greater than the backscatter signal level. The large reflected signals overdrive the optical receiver producing an output electrical pulse that has a slowly decaying trailing edge. This receiver transient tail may be caused by carrier diffusion or trapping effects in the photodetector. Amplifier transient responses may also exhibit slowly decaying transients.
To view events in the fiber that would be hidden or distorted by the receiver transient tail, OTDR instrument makers have used optical modulators in the return optical signal path to mask the high amplitude reflections returning from the fiber under test. Data samples of the return reflected light from the fiber are acquired, processed, and displayed. An operator identifies the locations of the high amplitude reflections and manually sets mask locations corresponding to the reflections. With the mask locations set, the fiber is re-examined. The optical modulator blocks the return optical signal from the fiber at the mask locations identified by the operator. The masking function is triggered either by a counter/digital comparator system or an analog ramp/comparator during the cable re-examination. One drawback to such systems is that the number of mask points is limited by the amount of hardware required to implement the system. In addition, the mask points have to be set manually by the operator and then the cable has to be re-examined.
U.S. Pat. No. 4,769,534 to Brand and assigned to the assignee of the present invention describes a method of reducing optical detector storage time effects by using an optical modulator to mask the return reflected signal prior to sampling the output of the receiver. The modulator is turned on just before or concurrent with the beginning of a sample strobe to allow the return reflected signal to impinge on the detector. This prevents a previous large return reflected signal from reaching the optical detector prior to the sample time and creating storage effects that might distort the electrical output signal during the sampling time.
One problem with the use of optical modulators, such as Bragg Cells, in these masking systems is that the modulators have a finite on/off ratio. This means that a certain amount of light passes through the cell during the time the device is masking the optical signal. Depending on the on/off ratio of the device, imperfect masking may result in a reduction but not an elimination of the distortion due to receiver transients. What is needed is a method for increasing the effectiveness of the masking performed with optical modulators having limited on/off ratios. Additionally, an automatic masking process is needed to replace the manual masking process.